Adamantane (tricyclo[3,3,1,13,7]decane, C10H16) is a colorless, non-toxic, crystalline compound having a structure of highly symmetrically cyclic tetrahedron. The chemical and physical properties of adamantane are as follows: melting point of 268° C., good heat stability and lubricity, ease of sublimation, good solubility in an organic solvent, and poor solubility in water.
Since hydrogen atoms in adamantane are easily substituted, adamantane can be converted into various derivatives thereof through a reaction such as bromination, oxidation, or alkylation. As adamantane and derivatives thereof may be used to produce fragrances, persistent pesticides, dyes, medicines, etc., they have wide potential use in the fields of medicine, textile, functional polymers, lubricants, surfactants, catalysts, photosensitive materials, etc. However, high manufacturing costs limit the application and development of adamantane.
In general, adamantane is produced via isomerization of tetrahydrodicyclopentadiene (referred to as THDCPD hereinafter and including exo-THDCPD and endo-THDCPD). THDCPD is produced via catalytic hydrogenation of dicyclopentadiene (a petroleum by-product, referred to as DCPD hereinafter, and including endo-DCPD and exo-DCPD), and most of the THDCPD produced in this manner is endo-THDCPD.
The scheme for producing a damantane is shown below:

Conventional methods for producing adamantane via isomerization of THDCPD include an aluminum (III) chloride method, a solid acid method, a superacid method, and an acidic ionic liquid method.
The aluminum (III) chloride method is performed by dissolving endo-THDCPD in a solvent and subjecting endo-THDCPD to isomerization directly using a solid catalyst of AlCl3 under proper reaction conditions to form adamantane. The yield of adamantane is about 15-20% (see, for example, Schleyer, J. Am. Chem. Soc., (1957), 79, 3292). There are various modifications of the aluminum (III) chloride method. For example, CN 101407442A discloses addition of a co-catalyst, such as sodium carbonate and sodium chloride, to the catalyst of AlCl3, and CN 1340483A discloses addition of a co-catalyst, such as alcohol, ether, ester, acid, and C2-C10 alkyl halide, to the catalyst of AlCl3. Although the yield of adamantane is larger than 50% in CN 101407442A and CN 1340483A, there are the following disadvantages: (1) a large amount of tar may be produced so that a complicated refining process is required to purify the resultant adamantane; (2) a large amount of the catalyst of AlCl3 is required and recycling of AlCl3 is difficult as AlCl3 is dispersed in the whole reaction system after the isomerization; and (3) separation of AlCl3 from the resultant adamantane requires use of an alkaline liquid followed by washing with a large amount of water, which produces a considerable amount of waste, resulting in environmental problems.
CN 101125791A discloses a solid catalyst system in which AlCl3 is supported on a molecular sieve. Although a liquid product may be easily separated from the solid catalyst system, the solid catalyst system need to be used under high hydrogen pressure (for example, 2.5 MPa) and at elevated temperature (for example, 160° C.)., and the yield of adamantane is relatively low (for example, a 30% yield).
The solid acid method is proposed to solve the problems that the AlCl3 catalyst cannot be reused and a considerable amount of waste catalyst need to be further treated. Solid acid catalysts have been industrially used by Idemitsu Petrochemical Co., Ltd. to produce adamantane. In U.S. Pat. No. 3,944,626 and US 20030018226 owned by Idemitsu, there is disclosed use of a solid acid catalyst, in which a metal selected from Pt, Re, Co, Ni, Fe, Cu, Ge, and the like is supported on zeolite by means of an ion exchange method, for isomerizing THDCPD at a temperature of 250□ and under a hydrogen pressure of 1-3 MPa to form adamantane along with a ring-opened product (an isomer of C10H18), and the yield of adamantane is up to about 30%. Additionally, CN 1935756 discloses an adamantane synthesizing method wherein a full-silicon medium-hole molecular sieve solid acid, which is surface-treated by an inorganic acid, is used as a catalyst for producing adamantane at a temperature of 250° C. under a hydrogen pressure of 1 Mpa. Although the solid acid catalyst may be reused, the synthesizing method needs to be performed at an elevated temperature and under a high hydrogen pressure, and a large amount of by-products may be produced.
In the superacid method, a superacid catalyst is used for isomerizing THDCPD into adamantane. Adamantane may be advantageously obtained when the acidity of the superacid catalyst is enhanced. US 20010051755 discloses a process for producing adamantane in which a HF—BF3 catalyst added with a co-catalyst such as platinum/activated carbon is used. When the process is performed at a temperature of 50° C. and under a hydrogen pressure of 1.5 MPa, the conversion rate of THDCPD is above 78%, and the selectivity of adamantane is above 88%. Additionally, it is disclosed in Synthesis, (1973), 488 and J. Org. Chem. (1984), 49, 4591 that THDCPD is isomerized at a temperature of 100° C. using a superacid catalyst of B (OSO2CF3)3—HSO3CF3 to produce adamantane at a yield of 47-64%. Although superacid catalysts has high activity and high selectivity so that the superacid method for producing adamantane has a better effect, they are highly corrosive. Furthermore, the technologies related to mass production of superacid catalysts and to equipment for producing adamantane using superacid catalysts are still immature, and thus the superacid method is unsuitable for industrial use.
The recently developed acidic ionic liquid method is disclosed in, for example, U.S. Pat. No. 7,488,859. An acidic ionic liquid has many advantages. For example, it is usually not miscible with adamantane so that recycling thereof and purification of adamantane become easier. In addition, the pH value of the acidic ionic liquid can be adjusted by modifying the proportions of the components contained therein. When the acidic ionic liquid is used as a catalyst for producing adamantane, there are advantages of high selectivity, easy operation, facile separation from adamantane, etc. However, the conversion rate for the isomerization of THDCPD is relatively low, and thus the yield of adamantane is unsatisfactory.